Organic light emitting devices (OLEDs) include several organic layers in which at least one of the layers includes an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Phys. Lett. 1987, 51, 913. Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor, U.S. Pat. No. 5,707,745.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was disclosed in U.S. Pat. No. 5,703,436, in which the TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg—Ag-ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg—Ag-ITO layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked, transparent, independently addressable, organic layer or layers, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
U.S. Pat. No. 5,707,745 disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. U.S. Pat. No. 5,707,745, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices. Until recently, it was not believed that organic materials could be used to produce efficient room temperature electrophosphorescence. In contrast, use of fluorescent dyes in OLEDs has been known for much longer, (C. H. Chen, J. Shi, and C. W. Tang, “Recent developments in molecular organic electroluminescent materials,” Macromolecular Symposia, 1997, 125, 1-48; U. Brackmann, Lambdachrome Laser Dyes (Lambda Physik, Gottingen, 1997, and references cited therein) and fluorescent efficiencies in solution approaching 100% are not uncommon. (C. H. Chen, 1997, op. cit.) Fluorescence is also not affected by triplet-triplet annihilation, which degrades phosphorescent emission at high excitation densities. (M. A. Baldo, et al., “High efficiency phosphorescent emission from organic electroluminescent devices,” Nature, 1998, 395, 151-154). Consequently, fluorescent materials are suited to many electroluminescent applications, particularly passive matrix displays.
An advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an ETL), which are in fact predominantly triplets in an OLED, may participate in energy transfer and luminescence in certain electroluminescent materials. In contrast, only a small percentage of excitons in fluorescent emitting devices, which are singlet-based, result in fluorescent luminescence. Fluorescence is, thus, at best only one-third as efficient as phosphorescence due to the formation of three times more triplet excitons than singlet excitons. Recently it was discovered that very high efficiency organic light emitting devices could be fabricated based on electrophosphorescence (M. A. Baldo, D. F. O'Brien, M. E. Thompson and S. R. Forrest, Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Applied Physics Letters, 1999, 75, 4-6.) The terms fluorescence and phosphorescence refer to the type of radiative emission, respectively, that is typically understood by one skilled in the art. That is, fluorescence refers to radiative emission from an excited singlet state and phosphorescence refers to radiative emission from an excited triplet state.
In view of the improved external efficiency that can be realized for electrophosphorescent OLEDs, it would be desirable to find additional materials as host materials for emissive phosphorescent dopant materials so that even higher external efficiencies can be produced.